Method and device for controlling power output of engine for working machine

ABSTRACT

An engine control device detects the state of work of a working vehicle such as a construction machine or the like, and controls the power output capacity of an engine automatically. A determination is made as to whether excavation or uphill traveling is being performed, based upon the detection signals from a hydraulic oil pressure detector for a hydraulic cylinder of an arm, detectors for arm and bucket operation commands, a shift operation detector for a transmission, a pitch angle detector for the vehicle body, a traveling acceleration detector, and an accelerator opening degree detector. When the result of this determination is that excavation or uphill traveling is being performed, the engine is controlled to operate at a high power capacity, while at other times it is controlled to operate at a low power output capacity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. Ser. No.10/570,170, filed Mar. 1, 2006, which is a National Phase filing ofPCT/JP2004/12641, filed Sep. 1, 2004, and is based upon and claims thebenefit of priority from Japanese Patent Application No. 2003-309533,filed Sep. 2, 2003, the entire contents of all of which are incorporatedby reference.

TECHNICAL FIELD

The present invention relates to a method and a device for controllingthe power output of an engine for a working vehicle.

BACKGROUND ART

For a working vehicle such as a construction machine or the like,techniques have been known from the past of changing over the poweroutput performance of an engine according to the work load (for example,see Patent References #1 and #2). According to these techniques, theworking vehicle is provided with a heavy work mode in which it ispossible to obtain power output up to a high level, and with a lightwork mode in which it is only possible to obtain power output up to alow level.

The driver selects between these modes by hand by operating a changeoverswitch. In other words, if he determines that the work which will beperformed henceforward is heavy work, then he selects the heavy workmode, while if he determines that it is light work, then he selects thelight work mode.

At this time, a controller which controls the engine controls the poweroutput capacity of the engine according to a command from the changeoverswitch. In other words, for the light work mode, it limits the poweroutput range of the engine to be less than or equal to a predeterminedvalue which is lower than its rated power output, for example bylimiting the fuel supply amount. By contrast, for the heavy work mode,it does not impose the above described limitation, so that it ispossible for the power output of the engine to reach the rated poweroutput or the maximum power output.

Since, by doing this, a narrow low power output range is used duringlight work, accordingly the consumption of energy becomes small so thatthe fuel consumption is reduced. And since, during heavy work, nolimitation is imposed upon the power output of the engine, it ispossible to obtain sufficient power output for performing the worksmoothly.

-   Patent Reference #1: Japanese Patent Laid-Open Publication Heisei    08-218442.-   Patent Reference #2: Japanese Patent Laid-Open Publication Heisei    11-293710.

With a certain type of working vehicle, it often happens that thevehicle does not continuously perform only heavy work or only lightwork, but rather performs heavy work and light work alternately during aseries of work processes. For example, with a wheel loader, duringtypical excavation and loading work, the vehicle successively performsprocesses such as approaching the subject material for work (which islight work), excavating the subject material for work and loading itinto the bucket (which is heavy work), and dumping of the subjectmaterial for work upon a transportation vehicle such as a dump truck orthe like (which is light work).

With this type of working vehicle or working process, in order to enjoythe benefits of the prior art technique to the maximum level, the drivermust operate the changeover switch while changing over between heavywork and light work. However, it is very burdensome to perform suchswitch operation frequently during the working process. As a result, itoften happens that work is performed with the changeover switch alwaysleft fixed in the heavy work mode, so that it is not possible toanticipate any reduction in the fuel consumption. On the other hand, ifemphasis is placed upon the fuel consumption, and work is performed withthe changeover switch left fixed in the light work mode, then it is notpossible to obtain sufficient power output during heavy work such asexcavation, and there is a fear that the working efficiency will bedeteriorated.

DISCLOSURE OF THE INVENTION

The objective of the present invention is, for a working vehicle such asa construction machine or the like, to control the power output capacityof the engine automatically according to the working state.

According to the present invention, one or a plurality of variablevalues relating to a state or states of one or a plurality of work loadswhich consume power output from the engine is detected, and the poweroutput capacity of the engine is controlled based upon the detectedvariable value. As a result, it is possible to control the power outputcapacity of the engine automatically according to the state of working.

A work apparatus such as, for example, an arm and/or a bucket may beincluded in the above work loads. As the variable value relating to thestate of the work apparatus, for example, it is possible to employ thehydraulic oil pressure of a hydraulic cylinder for operating the abovework apparatus, the type of operation which is performed with respect tothe above work apparatus, or the position or attitude of the above workapparatus.

Or a travel apparatus consisting of, for example, wheels and atransmission may be included in the above work loads. As a variablevalues relating to the state of the travel apparatus, for example, it ispossible to employ the type of gearshift operation which is performedwith respect to a transmission, a speed stage which is selected at thetransmission, the pitch angle in the longitudinal direction (i.e. in thetraveling direction) of the vehicle body, or the vehicle travelingspeed, or the traveling acceleration which corresponds to the openingdegree of the accelerator pedal.

As one example of control, it is possible to determine whether or notexcavation is being performed, based upon the result of detecting thevalue of a specific variable relating to the state of the work apparatusor of the travel apparatus. According to the result of thisdetermination, it is possible to perform control of the power outputcapacity so that the upper limit output torque curve in the case that itis determined that excavation is not being performed is lower than theupper limit output torque curve in the case that it is determined thatexcavation is being performed.

As another example of control, it is possible to determine whether ornot uphill traveling is being performed, based upon the result ofdetecting the value of a specific variable relating to the state of thetravel apparatus. According to the result of this determination, it ispossible to perform control of the power output capacity so that theupper limit output torque curve in the case that it is determined thatuphill traveling is not being performed is lower than the upper limitoutput torque curve in the case that it is determined that uphilltraveling is being performed.

As yet another example of control, based upon the result of detectingthe values of specific variables relating to the states of the workapparatus and of the travel apparatus, it is possible to perform adetermination as to whether or not excavation is being performed, and,in parallel, to perform a determination as to whether or not uphilltraveling is being performed. And, if it is determined that neitherexcavation nor uphill traveling is being performed, it is possible toperform control of the power output capacity so that the upper limitoutput torque curve is lower than the upper limit output torque curve inthe case that it is determined that at least one of excavation anduphill traveling is being performed.

And, as still another example of control, based upon the result ofdetecting the values of specific variables relating to the states of thework apparatus and of the travel apparatus, it is possible to perform adetermination as to which one of various different types of workprocesses is being performed. And it is possible to control the poweroutput capacity of the engine so that the upper limit output torquecurve is different according to which work process is determined.

And, as even yet another example of control, based upon the result ofdetecting the value of a specific variable relating to the state of thework apparatus or of the travel apparatus, it is possible to perform adetermination as to the magnitude of power output which the work load,i.e. the work apparatus or the travel apparatus, will demand. And it ispossible to control said power output capacity either stepwise orcontinuously, according to the result of this determination.

In a preferred embodiment, a determination is made as to whether or notthe excavation process is being performed, based upon the detected valueof the pressure of a hydraulic cylinder which drives the work apparatus.Since the hydraulic cylinder pressure of the work apparatus changes insensitive response to the starting and the stopping of the excavationprocess, the reliability of this determination about the excavationprocess is high. And, when the excavation process is being performed,the engine is controlled to operate in a high output mode which canmanifest the full high power output capacity which the engine reallyhas. On the other hand, during any process other than the excavationprocess, the engine is controlled to operate in a low output mode inwhich the power output capacity is limited to be lower than in the highoutput mode. In this low output mode, the engine is limited so that itsupper limit output torque curve becomes one which is defined bymultiplying the upper limit output torque curve for the high output modeby a predetermined coefficient which is less than 1.

And, in a preferred embodiment, further, a determination is made as towhether or not the vehicle is currently traveling uphill, based upon thedetection result for the pitch angle of the vehicle body while it istraveling, or for the traveling acceleration which depends upon theaccelerator pedal opening degree. And the engine is controlled tooperate in the above described high output mode, not only during theexcavation process, but also while uphill traveling is being performed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a wheel loader;

FIGS. 2A-2H are explanatory figures showing examples of excavation andloading working processes by the wheel loader;

FIG. 3 is a system diagram of an engine control device;

FIG. 4 is a flow chart showing a summary of an engine control procedure;

FIG. 5 is a graph showing an example of a curve of upper limit outputtorque against power output capacity of the engine, in a high outputmode and in a low output mode;

FIG. 6 is another graph showing an example of upper limit output torqueagainst power output capacity of the engine, in a high output mode andin a low output mode;

FIG. 7 is a side view showing the state of the work apparatus of thewheel loader during the excavation process;

FIG. 8 is a graph showing the change of the bottom pressure of a liftcylinder;

FIG. 9 is a flow chart showing a control procedure which determines thestart of the excavation process;

FIG. 10 is a flow chart showing a control procedure which determines theend of the excavation process;

FIG. 11 is a side view showing an excavation position of the workapparatus;

FIG. 12 is a flowchart showing a summary of a control procedure forselecting high output mode when, during the excavation process, thevehicle is moving up a slope; and

FIG. 13 is a figure for explaining a determination as to which processis currently being performed, and control of changeover of the outputmode of the engine.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, several embodiments of the control device and thecontrol method for an engine of a working vehicle according to thepresent invention will be described in detail with reference to thedrawings.

In the following disclosure, a wheel loader is taken as an example of aworking vehicle, and embodiments of the present invention forcontrolling the power output of an engine of a wheel loader areexplained. However, this wheel loader is shown as an example forexplanation of the present invention; the present invention could alsobe applied to engine power output control of various other types ofworking vehicle.

FIG. 1 is a side view of a wheel loader 1, which is an example of aworking vehicle.

As shown in FIG. 1, this wheel loader 1 comprises a rdivers cab 2, avehicle rear body portion 5 comprising an engine room 3 and rear wheels4, 4, and a front frame portion 7 comprising front wheels 6, 6. A workapparatus 10 is fitted to the front frame portion 7.

A base end portion of a lift arm 11 of this work apparatus 10 is freelypivotally attached to the front frame portion 7. The front frame portion7 and the lift arm 11 are coupled together by a pair of lift cylinders13, 13, and the lift arm 11 is pivoted by these lift cylinders 13, 13extending and retracting. A bucket 12 is freely pivotally attached tothe end portion of the lift arm 11.

A tilt arm 14 is freely rotatably supported upon the lift arm 11, almostat its central portion. One end portion of this tilt arm 14 and thefront frame portion 7 are coupled together by a tilt cylinder 15, andthe other end portion of the tilt arm 14 and the bucket 12 are coupledtogether by a tilt rod 16. When the tilt cylinder 15 extends andretracts, the bucket 12 pivots.

At the vehicle rear body portion 5, there are mounted a travel apparatus20 for propelling this wheel loader 1, and an engine 21 which suppliesits output power to the travel apparatus 20. The travel apparatus 20comprises a torque converter 22, a transmission 23 which is capable ofchanging over between forward and reverse traveling and is also capableof changing between a plurality of speed change stages, a differential24, speed reduction devices 25, 25 which drive the rear wheels 4 and thefront wheels 6, and the like. The power output of the engine 21 istransmitted to the differential 24 via the torque converter 22 and thetransmission 23 in order, and from there is distributed to the rearwheels 4 and the front wheels 6.

At the vehicle rear body portion 5, there is also mounted a variablecapacity hydraulic pump 26 which supplies pressurized hydraulic fluid tothe above described lift cylinders 13 and tilt cylinder 15. Thisvariable capacity type hydraulic pump 26 is driven using a portion ofthe power outputted from the above described engine 21.

Within the rdivers cab 2 there is provided a maneuvering device 30 whichincludes a transmission shift lever, an accelerator pedal, and a brakepedal, and which also includes levers or the like which operate the liftcylinders 13 and the tilt cylinder 15, all of which are operated by thedriver. By the driver operating this maneuvering device 30, it ispossible for him to perform changing over of the wheel loader 1 betweenforward and reverse traveling, adjustment of its traveling speed(acceleration and deceleration), and operation of the work apparatus 10(i.e. of the lift cylinders 13 and of the tilt cylinder 15).

Furthermore, in the vehicle rear body portion 5, there is also mountedan engine control device (omitted from FIG. 1) for controlling thedriving of the lift cylinders 13 and of the tilt cylinder 15 in responseto operation signals from the maneuvering device 30, and for controllingthe power output capacity of the engine 21 according to the theory ofthe present invention. This engine control device will be described indetail hereinafter.

With this type of wheel loader 1, load components which consume thepower output of the engine 21 include work loads and parasitic loads. By“work loads” is meant components which demand power output from theengine 21 in order to perform tasks which apply work directly upon theexternal environment of the vehicle (such as excavation or raising ofthe ground or of a cliff, shifting of the vehicle itself, and so on).For example, the above described work machine 10, variable capacity typehydraulic pump 26, and travel apparatus 20 are included as work loads.By contrast, by “parasitic loads” is meant components which demand poweroutput from the engine 21 in order to operate internally to the vehiclewithout performing work. For example, an engine cooling system, an airconditioning system, a battery charging system and the like are providedand are included as parasitic loads, but none of these are shown in FIG.1.

The above described engine control device is endowed with the functionsof detecting the states of the work loads (for example, of the workmachine 10, of the variable capacity type hydraulic pump 26, and of thetravel apparatus 20) (hereinafter this will be termed the “workingstate”), and of controlling the power output capacity of the engine 21by inferring the level of power output which these work loads demand.When performing work which consists of a sequence of a plurality ofprocesses, the level of power output which the work loads demand oftenis different according to the process. For example it may happen that,among the sequence of processes, a comparatively high power output maybe demanded by a specific process, but a comparatively low power outputis sufficient for the other processes. In the following, the poweroutput control by the engine control device will be explained inconcrete terms by taking the example of excavation and loading work,which is a typical type of work for which the wheel loader 1 may beused.

FIGS. 2A to 2H show an example of a sequence of work processes for thewheel loader 1 to perform excavation and loading work.

The wheel loader 1 repeatedly performs a plurality of processes like thefollowing in order, in order to excavate subject material for work andto load this subject material for work into a transportation machinesuch as a dump truck or the like.

A forward traveling process (FIG. 2A): the driver drives the vehicle totravel forward towards the subject material for work while, by operatingthe lift cylinders 13 and the tilt cylinder 15, setting the workapparatus 10 into an excavation attitude in which, with the lift arm 11in a low position, the bucket 12 is oriented horizontally.

An excavation process: (FIGS. 2B and 2C): the driver drives the vehiclefurther forward and digs the blade edge of the bucket 12 into thesubject material for work (FIG. 2B: the digging in sub-process), andthen he operates the tilt cylinder 15 and tilts the bucket 12 back, sothat the subject material for work is scooped up into the bucket 12(FIG. 2C: the scooping up sub-process). According to the type of thesubject material for work, this scooping up sub-process sometimes can becompleted by simply tilting the bucket 12 backwards in one movement; orin some cases the operations of tilting the bucket 12 back, putting itinto neutral, and again tilting it back are repeatedly performed.

A reverse traveling and boom raising process (FIG. 2D): After havingscooped up the subject material for work into the bucket 12, whiledriving the vehicle to travel backwards, the driver extends the liftcylinders 13 so as to raise the lift arm 11.

A forward traveling and boom raising process (FIG. 2E): While drivingthe vehicle to travel forward and bringing it near to a dump truck, thedriver further extends the lift cylinders 13 and raises the lift arm 11,until the height of the bucket 12 reaches a loading height.

A soil dumping process (FIG. 2F): The driver dumps the bucket 12 at apredetermined position, thus loading the subject material for work uponthe load bed of the dump truck. This process often is performed whilecontinuing to travel forward from the previous forward traveling andboom raising process.

A reverse movement and boom lowering process (FIG. 2G): The driverlowers the lift arm 11 while driving the vehicle to travel backwards,thus returning the bucket 12 to the excavation attitude.

The above are typical processes which make up one cycle of theexcavation loading work.

Furthermore, in FIG. 2H, there is shown a simple traveling process inwhich the vehicle simply travels along. In this process, the driverdrives the vehicle to travel forward while keeping the lift arm 11 in alow position. It sometimes also is the case that a load is loaded intothe bucket 12 and the load is transported, and it is also sometimes thecase that the vehicle is driven without carrying any load.

The power output of the engine 21 which is required during these sixtypes of process is different for each process. In particular, duringthe excavation process (FIGS. 2B and 2C), a greater power output isrequired than during the other processes. Thus, it is desirable toperform a determination as to which process is currently beingperformed, and to perform control of the power output capacity of theengine 21 so as to be able to output the power which is required by thiscurrent process. In this case, t is possible to divide the abovedescribed six types of processes generally into two types of process,the excavation process and the other processes, and to control the poweroutput capacity of the engine 21 in two steps. Or, the above describedsix types of processes may be divided into three or more types, and thepower output capacity of the engine 21 may be controlled in three ormore steps. Or, preferably, it may be arranged to determine the level ofpower output which the work load demands by determining which process isbeing performed, and, according thereto, to vary the power outputcapacity of the engine 21 stepwise or continuously

Here by the power output capacity of the engine 21 is meant, the maximumlevel of power which the engine 21 has the capability or the capacity tooutput. Control of the power output capacity of the engine 21 maytypically be performed by, for example, a method of controlling theupper limit value of the amount of fuel injection to the engine 21. Forexample, if the upper limit value of the fuel injection amount is set tobe greater, then the power output capacity becomes higher; while, if theupper limit value of the fuel injection amount is set to be lower, thenthe power output capacity becomes lower. It is possible to display thepower output capacity using a torque curve, which shows the upper limitoutput torque which can be outputted according to the rotational speedof the engine 21. The higher is the upper limit output torque curve, thehigher does the power output capacity become.

FIG. 3 is a system diagram showing an example of an engine controldevice 40 which performs control of the power output capacity of theengine 21.

In FIG. 3, a controller 50 can be implemented by a computer whichcomprises a storage device which is used, for example, as a programmemory and a work memory, and a CPU which executes a program. Thiscontroller 50 is connected to a governor 27 which controls the poweroutput of the engine 21 by adjusting the amount of fuel injection whichis supplied to the engine 21 by a fuel injection pump not shown in thefigures. It becomes possible to vary the power output of the engine 21by outputting commands from the controller 50 to the governor 27, andvarying this fuel injection amount. The engine 21 drives the variablecapacity type hydraulic pump 26, which is one of its work loads.

A capacity control device 41 is connected to the variable capacity typehydraulic pump 26. Orders are outputted from the controller 50 to thecapacity control device 41, and thereby it becomes possible to vary thecapacity of the variable capacity type hydraulic pump 26. Upon adischarge circuit 42 of the variable capacity type hydraulic pump 26,there are provided a tilt operation valve 43 which is connected to thetilt cylinder 15, which is one of the work loads, and a lift operationvalve 44 which is connected to the lift cylinders 13, which similarlyconstitute one of the work loads.

A bottom pressure detector 45 is provided at the bottom end of one ofthe lift cylinders 13. (Here by the “bottom end” is meant the end of thecylinder at which increase of hydraulic oil pressure causes it toextend. The opposite end to the “bottom end” is termed the “head end” orthe “rod end”.) This bottom pressure detector 45 is, for example, apressure switch. The detection signal for the bottom pressure which isoutputted from the bottom pressure detector 45 is inputted to thecontroller 50.

Furthermore, the controller 50 is connected to a shift position detector31 which detects the position of a transmission shift lever which isincluded in the maneuvering device 30, and it inputs this detectionsignal for the shift position from the shift position detector 31,thereby detecting shift operation of the transmission 23 of the travelapparatus 20 (or the speed stage which has been selected by softwareoperation), which is one of the work loads. When the transmission 23 isa transmission 23 which has, for example, four forward traveling speedstages (F1 to F4) and two reverse traveling speed stages (R1 and R2),then, based upon this shift position detection signal, the controller 50detects which speed stage among the speed stages F1 to F4, R1, and R2has been selected.

The controller 50 is further connected to an arm operation commanddetector 32 which detects the position of a lift cylinder operatinglever (a lift arm operating lever) included in the maneuvering device 30(in other words, which detects an arm operation command from thedriver), and inputs an operation command detection signal from this armoperation command detector 32. The controller 50 controls the liftoperation valve 44 based upon this operation command detection signal,so as to operate the lift arm 11. Furthermore, the controller 50 detectsthe type of operation (for example, raise, neutral, lower, or float)which is currently being performed for the lift arm 11, based upon thedetection signal from the arm operation command detector 32, or upon theoperation signal to the lift operation valve 44.

Furthermore, the controller 50 is connected to a bucket operationcommand detector 33 which detects the position (in other words, a bucketoperation command from the driver) of a tilt cylinder operating lever(i.e. a bucket operating lever) which is included in the maneuveringdevice 30, and inputs an operation command detection signal from thisbucket operation command detector 33. And, based upon this bucketoperation command detection signal, the controller 50 operates thebucket 12 by controlling the tilt operation valve 43. Moreover, thecontroller 50 detects the type of operation which is currently beingperformed for the bucket 12 (for example, tilt back, neutral, or dump),based upon the operation command detection signal from the bucketoperation command detector 33 or upon the operation signal to the tiltoperation valve 43.

The controller 50 further is connected to a speed meter 34 which detectsthe traveling speed of the vehicle, and inputs a traveling speeddetection signal from this speed meter 34.

The controller 50 controls the power output capacity of the engine 21,according to the theory of the present invention, based upon one or morevariable values among various variable values which indicate variouswork load states which have been detected, such as the shift operationof the transmission 23 (the speed stage which has been selected), thebottom pressure of the lift cylinder 13, the operation of the lift arm11, the operation of the bucket 12, the traveling speed of the vehicle,and the like. The procedure for this control will now be explained inthe following.

In FIG. 4, a summary of an example of a control procedure for the outputcapacity of the engine 21 is shown by a flow chart.

By the control shown in FIG. 4, it is determined, based upon the valuesof specified variables which indicate the above described workingstates, whether or not the process which is currently being performed isa predetermined process in which particularly high power output isdemanded (for example, the excavation process). Based upon the result ofthis determination, one or the other of two types of control modes whichhave been set up in advance, a “low output mode” and a “high outputmode”, is selected as the mode for controlling the power output capacityof the engine. The concrete meanings of these two types of control modeswill be explained hereinafter.

As shown in FIG. 4, in a step S11, the controller 50 first outputs acommand to the governor 27 simultaneously with the starting of theengine 21, and drives the engine 21 in the low output mode. In this lowoutput mode, the controller 50 commands the governor 27 by a method suchas, for example, limiting the upper limit value of the range ofvariability of the injection amount (mg/stroke) of a fuel injection pumpnot shown in the figures to a predetermined low value, or the like, soas to limit the power output capacity of the engine 21 to be lower thanin the high output mode.

And, in a step S12, the controller 50 performs an excavation startdetermination which will be described hereinafter, and determineswhether or not the wheel loader 1 is performing the excavation process(from FIG. 2A described previously to FIG. 2B). If the result of thestep S12 is that excavation work is not being performed, then the flowof control returns to the step S11.

If the result of the step S12 is that excavation work is beingperformed, then the flow of control proceeds to a step S13, and thecontroller 50 outputs a command to the governor 27 to drive the engine21 in the high output mode. In the high output mode, the controller 50raises the power output capacity of the engine 21 to be higher than inthe low output mode (for example, it enables the engine 21 to manifestthe maximum power output which it basically possesses), by canceling theabove described limitation with regard to the injection amount of thefuel injection pump, thus raising the upper limit value of the range ofvariability of the fuel injection amount to be higher than in the lowoutput mode, or the like.

And, in a step S14, the controller 50 performs a excavation completeddetermination which will be described hereinafter, and determineswhether or not the excavation process by the wheel loaded 1 has beencompleted. If the result of the step S14 is that the excavation processhas not been completed, then the flow of control returns to the stepS13. Furthermore, if the result of the step S14 is that the excavationprocess has been completed, then the flow of control returns to the stepS11, and the engine 21 is again driven in the low output mode.

In the following, each of the steps of the flow chart shown in FIG. 4will be explained in detail.

First the high output mode and the low output mode of the engine 21,shown in the steps S11 and S14, will be explained. In FIG. 5, there isshown an example of a torque curve showing the power output capacity ofthe engine 21 in a high output mode and in a low output mode. In FIG. 4,the horizontal axis is the rotational speed of the engine 21, and thevertical axis is the output torque. Furthermore, the curve 29 is amatching curve of the torque converter 22.

In FIG. 5, the solid line 28A indicates the curve of the upper limitoutput torque of the engine 21 in the high output mode in which it hasbeen arranged to obtain a high power output without imposing anylimitation upon the power output capacity of the engine 21, and this,for example, corresponds to the rating or to the maximum power output ofthe engine 21. Although the output torque of the engine 21 can be variedaccording to the adjustment of the fuel injection amount by operation ofthe accelerator pedal by the driver, its range of variability is at orbelow the upper limit output torque curve 28A.

On the other hand, the broken line 28B indicates the curve of the upperlimit output torque of the engine 21 in the low output mode in which ithas been arranged to obtain only a lower power output than in the highoutput mode. This upper limit output torque curve 28B for the low outputmode is limited so as to be the upper limit output torque curve 28A inthe high output mode, multiplied by a coefficient α (where α<1) (forexample, 80%). In the low output mode, the range of variability of theengine output is limited to a low range at or below the upper limitoutput torque curve 28B. The converse of this is the benefit that it ispossible to economize upon fuel. The controller 50 executes the lowoutput mode by a method such as controlling the governor 27 so as toimpose a limitation upon the fuel injection amount (for example, makingthe upper limit value of the fuel injection amount to be lower thanduring the high output mode), or the like.

Furthermore, in FIG. 6 there is shown a torque curve which indicates thepower output performance of the engine 21, according to a differentexample of control. In FIG. 6, the broken line 28C indicates the upperlimit output torque curve in the low output mode, and this means thatthe output torque range of variability is limited to be yet lower thanthe upper limit output torque curve 28B in the low output mode shown inFIG. 5 and described above. In this manner, there are various variationswith regard to what type of torque curve should be used in practice whenlimiting the power output performance of the engine. It would beacceptable to employ only one of the upper limit output torque curves28B and 28C for the low output mode shown in FIG. 5 and FIG. 6, or itwould also be acceptable to use one or the other of these upper limitoutput torque curves 28B and 28C according to the current conditions.

FIG. 7 is a side view showing a state in which the wheel loader 1 isexcavating a subject material for work with the bucket 12.

As shown in FIG. 7, during the excavation process, the vehicle is drivenforward in the direction of the arrow sign A and the blade edge of thebucket 12 is dug into the subject material for work Z, and the bucket 12is tilted backwards. During this operation, forces act upon the bucket12 in the directions of the arrow sign B and of the arrow sign C. As aresult, high pressures are created at the bottom ends of the liftcylinders 13 and the tilt cylinder 15 (their ends at which increase ofhydraulic oil pressure causes them to extend). Furthermore, dependingupon the work attitude, a force may act upon the bucket 12 in thedirection of the arrow sign D, and in this case a high hydraulic oilpressure is created at the head end (the rod end) of the tilt cylinder15.

The magnitudes of these pressures clearly vary during the excavationprocess and during the other processes. Accordingly, it is possible todetermine whether or not the excavation process is being performed fromat least one of the magnitudes of these pressures. For example, it ispossible to determine whether or not the excavation process is beingperformed, by comparing the pressure at the bottom ends of the liftcylinders 13 (hereinafter termed the bottom pressure of the liftcylinders 13) with a reference value which is determined in advance. Or,it is also possible to determine whether or not the excavation processis being performed, by comparing the pressure at the bottom end of thetilt cylinder 15 (hereinafter termed the bottom pressure of the tiltcylinder 15) with a reference value which is determined in advance.

FIG. 8 is a graph showing an example of change of the bottom pressure ofthe lift cylinders 13 in each of the processes during the excavation andloading work of the wheel loader 1 shown in FIG. 2A through FIG. 2G. InFIG. 8, the bottom pressure of the lift cylinders 13 is shown along thevertical axis, while time is shown along the horizontal axis.

As shown in FIG. 8, the bottom pressure of the lift cylinders 13 isquite low during the forward movement process (FIG. 2A); when theexcavation process (FIGS. 2B and 2C) starts, it precipitously risesconsiderably; it continues to be quite high over the entire period ofthe excavation process (FIGS. 2B and 2C); and, when the excavationprocess terminates, it abruptly drops considerably. Now, if a pressure Pis set as a reference value as shown in FIG. 8, the bottom pressure ofthe lift cylinders 13 is lower than the reference value P over theentire period of the forward movement process, and is higher than thereference value P over the entire period of the excavation process, sothat this difference is very clear.

Furthermore, in the first half of the reverse movement and boom raisingprocess (FIG. 2D), the forward movement and boom raising process (FIG.2E), and the soil dumping process (FIG. 2F), the bottom pressure of thelift cylinders 13 is higher than the reference value P, and thereafterit is lower than the reference value P. The time for the forwardmovement process is normally some seconds (for example five seconds).Accordingly, the bottom pressure of the lift cylinders 13 is lower thanthe predetermined pressure P over a predetermined time period (forexample one second), and thereafter rises, and, when the time point atwhich it exceeds the reference value P is detected, it is possible todetermine that this time point is the time point of the starting of theexcavation process.

In the following, the control for detecting the time point at which theoperation process starts will be explained in concrete terms using theflow chart of FIG. 9.

After the start of work, in a step S101, the controller 50 determineswhether or not the bottom pressure of the lift cylinders 13 is less thanor equal to the reference value P, based upon the result of detection bythe bottom pressure detector 45. If the result of this step S101 is NO,then the flow of control returns to before the step S101. If the resultof this step S101 is YES, then the flow of control proceeds to a stepS102, and the controller 50 starts time measurement.

In a step S103, the controller 50 makes a determination as to whether ornot the state in which the bottom pressure of the lift cylinders 13 isgreater than or equal to the reference value P has continued over atleast a predetermined time period (for example, one second). If theresult of this step S103 is NO, then the flow of control returns tobefore the step S103. If the result of this step S103 is YES, then theflow of control proceeds to a step S104, and the controller 50 makes adetermination as to whether or not the bottom pressure of the liftcylinders 13 exceeds the reference value P. If the result of this stepS104 is NO, then the flow of control returns to before the step S104. Ifthe result of this step S104 is YES, then the flow of control proceedsto a step S105, and the controller 50 determines that the excavationprocess has started.

Next the control for the excavation completed determination shown in thestep S14 of FIG. 4, in which it is determined whether or not theexcavation process has been terminated, will be explained.

The fact that the excavation process has terminated may be determinedusing the determination conditions A1, A2, and A3 as described below,based upon the direction of shifting of the vehicle during each of theprocesses of the excavation and loading work shown in FIG. 2, and uponthe changes of the bottom pressure of the lift cylinders 13 during eachof the processes shown in FIG. 8.

The determination condition A1: after the start of the excavationprocess, the transmission 23 is changed over from forward traveling toneutral or to reverse traveling.

The determination condition A2: After the start of the excavationprocess, the bottom pressure of the lift cylinders 13 has dropped belowthe reference value P, and thereafter has maintained a state lower thanthe reference value P over a predetermined time period (for example, onesecond).

By the way, even if during the control for determining the start ofexcavation shown in FIG. 9 it has been temporarily determined in thestep S105 that the excavation process has started, it may be sometimesbe the case that actually the excavation process is not being performed.For example, a case such as when the vehicle collides with the subjectmaterial for work Z and suddenly moves downwards, so that the bottompressure of the lift cylinders 13 exceeds the reference value P onlyinstantaneously, or a case when during forward traveling, due toirregularities upon the surface of the road, a shock is instantaneouslygenerated and applied to the work apparatus 10, or the like, maycorrespond to this. Since, in this type of case, the excavation processhas not yet actually started, it is necessary to return the engine 21from the high output mode to the low output mode. In order to performthis control, after having determined that the excavation process hasstarted, the continuous time period is measured in which the bottompressure of the lift cylinders 13 exceeds the reference value P, and ifthis continuous time period does not exceed a predetermined time period,it is determined that the excavation process has terminated (i.e., hasnot yet started). This is termed the determination condition A3.

In this embodiment, if any one among the above described determinationconditions A1 to A3 has been satisfied, it is determined that theexcavation task has terminated.

In the following, the control for detecting that the excavation processhas terminated will be explained in concrete terms, using the flow chartshown in FIG. 10.

First, the determination condition A1 will be explained. In a step S109,the controller 50 inputs the detection signal from the shift positiondetector 31 (FIG. 3), and determines whether or not the transmission 23is in the neutral or the reverse traveling position. If the result ofthe step S109 is NO, then the flow of control returns to before thisstep S109. But if the result of the step S109 is YES, then the flow ofcontrol proceeds to a step S110, and the controller 50 determines thatthe excavation process has terminated.

Next, the determination condition A2 will be explained. In a step S114,the controller 50 makes a determination as to whether or not the bottompressure of the lift cylinders 13 is less than or equal to the referencevalue P. If the result of the step S114 is NO, then the flow of controlreturns to before this step S114. But if the result of the step S114 isYES, then the flow of control proceeds to a step S115, and thecontroller 50 starts time measurement.

In a step S116, the controller 50 makes a determination as to whether ornot the time period which is being measured, in other words the timeperiod over which the bottom pressure of the lift cylinders 13 has beenin a state lower than the reference value P, has continued over a secondset time period which is determined in advance (for example 0.5seconds). If the result of the step S116 is NO, then the flow of controlreturns to before this step S116. But if the result of the step S116 isYES, then the flow of control proceeds to the step S110, and thecontroller 50 determines that the excavation process has terminated.

Next, the determination condition A3 will be explained. In a step S112,the controller 50 starts time measurement. In a step S113, thecontroller 50 makes a determination as to whether or not the time periodwhich is being measured, in other words the time period over which thebottom pressure of the lift cylinders 13 has continued to be in thestate higher than the reference value P, has exceeded a first set timeperiod (for example one second) which is determined in advance. If theresult of the step S113 is YES, then the flow of control returns tobefore the step S112. But if the result of the step S113 is NO, then theflow of control proceeds to the step S110, and the controller 50determines that the excavation process has terminated.

It should be understood that, as a variant embodiment, it would beacceptable to arrange to perform the determination condition A3 beforeperforming the determination conditions A1 and A2, and to arrange toperform the determination conditions A1 and A2 if the result of the stepS113 of the determination condition A3 is YES. Or, as another variantembodiment, it would also be acceptable to perform the determination ofthe determination condition A3 directly after the step S105 of theexcavation start determination shown in FIG. 9, and to omit thedetermination of the determination condition A3 in the control for theexcavation completed determination shown in FIG. 10.

As has been explained above, based upon the state of the bottom pressureof the lift cylinders 13, the controller 50 determines whether or notthe current process is the excavation process, and controls the poweroutput capacity of the engine 21 during the excavation process in thehigh output mode, while, during any process other than the excavationprocess, it controls the power output capacity of the engine 21 in thelow output mode. Since, by doing this, working is performed at a highpower output capacity during heavy work, and at a low power outputcapacity during other light work, accordingly it is possible to obtainonly the power output which is necessary for the work, and moreover itis possible to prevent useless high consumption of energy, so that thefuel consumption is reduced. Furthermore, it is possible to provide acomfortable operating feeling to the driver, since the output capacityof the engine 21 is automatically increased during the excavationprocess.

Moreover, it is determined that the excavation process has started whenthe bottom pressure of the lift cylinders 13 has exceeded the referencevalue P after it has been below the reference value P over thepredetermined time period. Due to this, the fear of mistaken detectionof the start of the excavation process during the reverse movement andboom raising process, during the forward movement and boom raisingprocess, and during the soil dumping process is reduced. Furthermore, ifit is only the case that the bottom pressure of the lift cylinders 13instantaneously or temporarily has exceeded the reference value P, thenthe determination that the excavation process has started is immediatelycanceled. Due to this, even if for example a mistaken determination hasbeen made that excavation work is under way and the engine 21 has beenoperated in the high output mode, it is possible to determine within ashort time period that this is a mistaken determination, so that it ispossible to prevent deterioration of the fuel consumption.

Yet further, after it is determined that the excavation process hasstarted, it is determined that the excavation process has terminatedwhen the transmission 23 is shifted to the neutral or to the reversetraveling position. Moreover, after it is determined that the excavationprocess has started, it is also determined that the excavation processhas terminated when the bottom pressure of the lift cylinders 13 becomesless than or equal to the reference value P, and this situation hascontinued for longer than the second time period which has beendetermined in advance. Due to this, the accuracy of detection of thetermination time point of the excavation process is high.

It should be noted that the bottom pressure of the lift cylinders 13 isused in the above described control for determining whether or not theexcavation process is currently being performed. However, instead of, orin addition to, this, it would also be acceptable to utilize the bottompressure of the tilt cylinder 15. For example, it would be possible todetermine that the excavation process has started, when, after thebottom pressure of the tilt cylinder 15 has been in the state of beingbelow a predetermined value over a predetermined time period, it hasexceeded that predetermined value. Moreover, since the power outputcapacity of the engine is changed over to the high output mode when theabove described bottom pressure has been raised to or above thereference value P by the bucket 12 digging into the subject material forwork, at the time point thereafter when operation to tilt the bucketstarts, the system is already in the high power output mode, so that nodelay occurs in raising the power output of the engine.

Even further, in the above described control, as the material fordetermining whether or not the excavation process is currently takingplace, it is possible to employ the force which is applied to anactuator of the work apparatus, such as the lift cylinders 13 or thetilt cylinder 15. However, this may also be performed using some otherfactor as material for the determination.

Now, a variant embodiment of control using such a different factor willbe explained. In other words, in the evacuation start determination, thefollowing determination conditions B1, B2, and B3 are used.

The determination condition B1: the transmission 23 is in the first orin the second forward traveling speed stage (F1 or F2).

The determination condition B2: the work apparatus 10 is in theexcavation position.

The determination condition B3: the vehicle traveling speed is less thanor equal to a set speed.

It is determined that the working vehicle is performing the excavationprocess, when at least one determination condition among thesedetermination conditions B1 to B3 is fulfilled.

Now, the excavation position of the above described determinationcondition B2 will be explained. FIG. 11 is a side view showing the workapparatus 10 in its excavation position.

As shown in FIG. 11, the base end portion of the lift arm 11 is freelypivotally attached to the front frame portion 7 via an arm pin 73, andthe front frame portion and the lift arm 11 are coupled together by thelift cylinders 13, 13. When the lift cylinders 13, 13 extend, the liftarm 11 pivots about the arm pin 73 as a center. The bucket 12 is freelypivotally attached at the end portion of the lift arm 11 via a bucketpin 76, and the front frame portion 7 and the bucket 12 are coupledtogether by the tilt cylinder 15 and by a link device 78. When the tiltcylinder 15 extends, the bucket 12 pivots about the bucket pin 76 as acenter.

By, for example, determining upon the line Y-Y which connects the armpin 73 and the bucket pin 76 (i.e. the line which specifies the attitudeor the position of the lift arm 11) as a reference line, the controller50 is able to determine that the work apparatus 10 is in the excavationposition, if the downwards dip of the reference line with respect to ahorizontal line passing through the arm pin 73 is greater than or equalto a predetermined value. And, as a method for ascertaining the attitudeor the position of this type of work apparatus 10, the controller 50may, for example, employ a method of calculation by using the stroke ofthe lift cylinders 13, 13 which is detected by a stroke sensor (notshown in the figures) which is fitted to the lift cylinders 13, 13, or amethod of calculation by using the angle of elevation of the liftcylinders 13, 13 which is detected by an angle sensor (not shown in thefigures) which is fitted to the lift cylinders 13, 13, or a method ofcalculation based upon the operation commands which are outputted fromthe controller 50 to the lift operation valve 44, or the like.

Next, a variant embodiment for the control will be described whichpermits a high power output to be obtained during a specified state ofworking other than the excavation process. The control according to thisvariant embodiment may be employed together with the above describedcontrol which selects the high output mode during the above describedexcavation process, or may also be employed instead thereof.

Here, a control example will be explained in which the high output modeis selected when the wheel loader 1 is performing the action of uphilltraveling upon inclined terrain.

It sometimes happens, when the wheel loader 1 is performing the actionof uphill traveling upon a slanted surface which has a slope anglegreater than or equal to a predetermined value, that a higher outputtorque is desired than the upper limit output torque which can beoutputted in the low output mode. Accordingly, in this embodiment, adetermination is made as to whether or not the vehicle is performing theaction of uphill traveling, and it is arranged to obtain a higher poweroutput if the vehicle is uphill traveling, than if it is not doing so.

Referring again to the previously described FIG. 3, to the vehicle bodyof this wheel loader 1 there are provided a pitch angle detector 46which measures the pitch angle of the vehicle body in its longitudinaldirection, an accelerator pedal opening degree detector 48 whichmeasures the opening degree of an accelerator pedal 49, and anacceleration detector 47 which measures the acceleration of the vehicle,and each of these is connected to the controller 50. Based upon theoutputs of these sensors 46 through 48, the controller 50 detects thepitch angle of the vehicle, the opening degree of the accelerator pedal,and the acceleration. With regard to the acceleration, instead of usingthe acceleration detector 47, it would also be acceptable to calculateit from the speed which has been detected by the speed meter 34. Thecontroller 50 determines that the vehicle is traveling uphill, if thevehicle is inclined at a pitch angle greater than or equal to a constantvalue and moreover is being driven. Furthermore the controller 50determines that the vehicle is traveling uphill, if an accelerationgreater than a predetermined value is not being obtained, although theaccelerator pedal is being opened up by greater than or equal to aconstant value. Or it may be determined whether or not the vehicle istraveling uphill, only from the accelerator pedal and the acceleration.

FIG. 12 is a flow chart showing the control procedure for the engine 21,according to this variant embodiment.

In a step S21, the controller 50 outputs a command to the governor 27simultaneously with the starting of the engine 21, and performslimitation of the power output by, for example, restricting theinjection amount of the fuel injection pump or the like, so that theengine 21 operates in the low output mode. And, in a step S22, thecontroller 50 performs the previously described excavation startdetermination, and determines whether or not the wheel loader 1 isperforming the excavation process. If the result of the step S22 is thatit is determined that the excavation process is not being performed,then the flow of control returns to the step S21. But, if the result ofthe step S22 is that it is determined that the excavation process isbeing performed, then the flow of control proceeds to a step S23, andthe controller 50 outputs a command to the governor 27, so as to returnto the state in which the above described limitation of the injectionamount of the fuel injection pump is not being imposed, so that it ispossible for the engine 21 to manifest its high power output capacityand to operate in the high output mode.

After having entered into operation in the high output mode, in a stepS24, the controller 50 performs the previously described excavationcompleted determination, and makes a determination as to whether or notthe excavation process by the wheel loader 1 has terminated. If theresult of the step S24 is that it is determined that the excavationprocess has not terminated, then the flow of control returns to the stepS23. But, if the result of the step S24 is that it is determined thatthe excavation process has terminated, then the flow of control returnsto the step S21, and the engine 21 operates in the low output mode.

Moreover, in parallel with the step S22, in a step S25, the controller50 performs the above described uphill traveling determination, anddetermines whether or not the wheel loader 1 is currently travelinguphill. If the result of the step S25 is that it is determined that thevehicle is not traveling uphill, then the flow of control returns to thestep S21. But, if the result of the step S25 is that it is determinedthat the vehicle is traveling uphill, then the flow of control proceedsto a step S26, in which the controller 50 outputs a command to thegovernor 27, and returns to a state in which the above describedlimitation upon the injection amount by the fuel injection pump is notimposed, so that the engine 21 is able to operate in its high outputmode in which it can manifest a high power capacity.

After having entered into operation in the high output mode, in a stepS27, the controller 50 performs the previously described uphilltraveling determination, and makes a determination as to whether or notthe wheel loader 1 is currently traveling uphill. If the result of thestep S27 is that it is determined that the vehicle is traveling uphill,then the flow of control returns to the step S26. But, if the result ofthe step S27 is that it is determined that the vehicle is not travelinguphill, then the flow of control returns to the step S21, and thecontroller 50 operates the engine 21 in the low output mode.

By the way, although, in the above described control, the engine 21operates in the same type of high output mode during excavation work andduring uphill traveling as well, the control is not to be considered asbeing limited by this feature. For example it would also be acceptable,during uphill traveling, to arrange to operate the engine 21 in anintermediate output mode, in which it has a power output capacity whichis intermediate between the low output mode and the high output mode.Or, it would also be acceptable to detect the magnitude of the load whenclimbing the slope, and to vary the power output capacity of the engine21 stepwise, or continuously, according to this magnitude of the load.Since, in either case, a sufficient power output can be obtained by theabove described type of control, not only during the excavation process,but also when traveling uphill, accordingly it is possible to performsmooth operation.

Next, a yet further variant embodiment for controlling the power outputcapacity of the engine will be explained.

With the control according to this variant embodiment, the controller 50determines which one among the processes shown in FIG. 2A through FIG.2H is currently being executed, based upon the result of detecting thework load of the wheel loader 1, and controls the power output capacityof the engine 21 based upon the result of this determination.

FIG. 13 shows in a table a method for determining which process iscurrently being performed in this control, and a method for controllingthe power output capacity of the engine according to the result of thatdetermination.

In the table shown in FIG. 13, in the uppermost line “work process”,there is given the name of the work process as shown in FIG. 2A throughFIG. 2H. Below this, in the lines “speed stage”, “work apparatusoperation”, and “work apparatus cylinder pressure”, there are shownvarious determination conditions which are used by the controller 50 fordetermining as to which of the processes is the current process.

In other words, in the line “speed stage”, determination conditions withregard to the speed stage of the transmission 23 are shown by circularmarks. Here, the case is assumed in which the transmission 23 has fourforward traveling speed stages F1 through F4, and two reverse travelingspeed stages R1 and R2. Furthermore, in the line “work apparatusoperation”, determination conditions with regard to operation of thework apparatus 10 by the driver are shown by circular marks. In otherwords, in the line “arm”, there is shown a determination conditionrelated to operation for the lift arm 11, while in the line “bucket”there is shown a determination condition related to operation for thebucket 12. Moreover, in the line “work apparatus cylinder pressure”,there are shown determination conditions related to the currenthydraulic oil pressure of a cylinder of the work apparatus 10, forexample the bottom pressure of the lift cylinder 11. Here, in relationto the lift cylinder bottom pressure, as shown in FIG. 8, apart from thepreviously described reference value P, three reference values A, B, andC which are higher than this reference value P are set in advance, and aplurality of pressure ranges are defined by these reference values A, B,and C (for example, a range less than the reference value P, a rangefrom the reference value A to the reference value C, a range from thereference value B to the reference value P, and a range below thereference value C), with these pressure ranges being set as the abovedescribed determination conditions.

By using a combination of the determination conditions “speed stage”,“arm”, “bucket”, and “work apparatus cylinder pressure” for each of theabove processes, the controller 50 becomes able to determine which ofthe processes is the process which is currently being performed.

In the line “engine output mode”, it is shown which of the “high outputmode” and the “low output mode” the controller 50 selects as a result ofthe above described determination. Below this, in the line “change ofoutput torque”, an example is shown in concrete terms of how the outputtorque of the engine 21 varies as a result of this type of engine poweroutput control.

The concrete operation of the controller 50 when performing the controlshown in FIG. 13 will now be explained in the following.

The controller 50 stores combinations of the determination conditions“speed stage”, “arm”, “bucket”, and “work apparatus cylinder pressure”corresponding to each of the processes shown in FIG. 13 in advance. Andthe controller 50 ascertains the speed stage (F1 through F4, or R1 orR2) of the transmission 23 which is currently selected, based upon thesignal from the shift position detector 31 shown in FIG. 3, ascertainsthe current type of operation for the lift arm 11 (for example, float,lower, neutral, or raise) based upon the signal from the arm operationcommand detector 32, and ascertains the current type of operation forthe bucket 12 (for example, dumping, neutral, or tilting), based uponthe signal from the bucket operation command detector 33. Moreover, thecontroller 50 ascertains the current bottom pressure of the liftcylinder 11, based upon the signal from the bottom pressure detector 45shown in FIG. 3.

And the controller 50 performs a so called matching procedure bycomparing the combination of the current speed stage, the current typeof operation of the arm, the current type of operation of the bucket,and the current lift cylinder bottom pressure which have thus beenascertained (in other words, the current working state) with thecombinations of determination conditions for “speed stage”, “arm”,“bucket” and “work apparatus cylinder pressure” corresponding to each ofthe processes which are stored in advance. As the result of thismatching procedure, the controller 50 determines which is the processcorresponding to the combination of determination conditions which bestmatches the current working state.

Now, in concrete terms, the combination of determination conditionscorresponding to each of the processes shown in FIG. 13 is as follows.

The forward movement process (FIG. 2A): In the speed stage F1, with boththe arm operation and the bucket operation in neutral, the workapparatus cylinder pressure is less than the reference value P. Withregard to the speed stage, it is possible to employ as the determinationcondition, not only simply that the current speed stage is F1 or F2, butalso that the speed stage has been changed over from F2 to F1, i.e. ashift down condition (since it often happens that the vehicle is drivenforward after having been first shifted down to F1 from being in thestage F2). Furthermore, it is also possible to employ, as a furtherdetermination condition related to the determination history, that theprocess which was determined upon directly before was the reversemovement and boom lowering process.

The excavation process (the digging in sub-process) (FIG. 2B): In thespeed stage F1 or F2, with the lift arm operation and the bucketoperation both at neutral, the work apparatus cylinder pressure is inthe range from the reference value A to the reference value C.Furthermore, it is also possible to employ, as a further determinationcondition related to the determination history, that the process whichwas determined upon directly before was the forward movement process.

The excavation process (the scooping up sub-process) (FIG. 2C): In thespeed stage F1 or F2, with the lift arm operation at “raise” or neutral,and with the bucket operation at “tilt”, the work apparatus cylinderpressure is in the range from the reference value A to the referencevalue C. Furthermore it would also be acceptable to add anotherdetermination condition with regard to the bucket operation, such as onewhich includes the case that alternate shifts between “tilt” and neutralis repeated (since, according to the subject material for work, itsometimes happens to repeat the operation of tilting back the bucket 12,then putting it into neutral, and then again tilting it back.).Furthermore it is also possible to employ, as a further determinationcondition related to the determination history, that the process whichwas determined upon directly before was the excavation digging insub-process.

The reverse movement and boom raising process (FIG. 2D): In the speedstage R1 or R2, with the lift arm operation at “raise”, and with thebucket operation at neutral, the work apparatus cylinder pressure is inthe range from the reference value B to the reference value P.Furthermore it is also possible to employ, as a further determinationcondition related to the determination history, that the process whichwas determined upon directly before was the excavation scooping upsub-process.

The forward movement and boom raising process (FIG. 2E): In the speedstage F1 or F2, with the lift arm operation at “raise” or neutral, andwith the bucket operation at neutral, the work apparatus cylinderpressure is in the range from the reference value B to the referencevalue P. Furthermore it is also possible to employ, as a furtherdetermination condition related to the determination history, that theprocess which was determined upon directly before was the reversemovement and boom raising process.

The soil dumping process (FIG. 2F): In the speed stage F1 or F2, withthe lift arm operation at “raise” or neutral, and with the bucketoperation at “dump”, the work apparatus cylinder pressure is in therange from the reference value B to the reference value P. Furthermoreit is also possible to employ, as a further determination conditionrelated to the determination history, that the process which wasdetermined upon directly before was the forward movement and boomraising process.

The reverse movement and boom lowering process (FIG. 2G): In the speedstage R1 or R2, with the lift arm operation at “float” or “lower”, andwith the bucket operation at “tilt”, the work apparatus cylinderpressure is in the range less than the reference value P. Furthermore itis also possible to employ, as a further determination condition relatedto the determination history, that the process which was determined upondirectly before was the soil dumping process.

The simple traveling process (FIG. 2H): In the speed stage F1, F2, F3,or F4, with the lift arm operation and the bucket operation both atneutral, the work apparatus cylinder pressure is in the range less thanthe reference value C.

The controller 50 determines which process is the current process byfinding out, from among the combinations of determination conditions foreach of the processes as described above, which one best matches thecurrent working state (it is possible to include, not only the currentworking state, but also, as described above, speed change shiftingoperation and changes of the operation of the work apparatus, or thehistory of previous determinations). And the controller 50 operates theengine 21 in the engine output mode which corresponds to the processwhich is determined upon. In other words, as shown in FIG. 13, if it isdetermined that the current process is the excavation process, thecontroller 50 operates the engine 21 in the high output mode. On theother hand, if it is determined that the current process is the forwardmovement process, the reverse movement and boom raising process, theforward movement and boom raising process, or the reverse movement andboom lowering process, then the controller 50 operates the engine 21 inthe low output mode. Furthermore, if it is determined that the currentprocess is the simple traveling process, then the controller 50, forexample, determines the magnitude of the load which is being imposedupon the vehicle by, as already explained, performing a determination asto whether or not the vehicle is traveling uphill or the like, andselects the low output mode or the high output mode, according to thismagnitude of the load. Since the power output capacity of the engine iscontrolled according to the result of a determination which is basedupon the combinations of transmission operation and work apparatusoperation by the driver, and the cylinder hydraulic oil pressure of thework apparatus and so on, accordingly it is possible to change the poweroutput capacity in conformance to the transition between processes.

As a result of the above described control, as shown in the lowermostline of FIG. 13, in the excavation and loading work, if the driver stepsupon the accelerator pedal during the excavation process, the outputtorque of the engine 21 is raised to the full torque which the engine 21is capable of outputting. However, during processes other than theexcavation process, even if the driver presses the accelerator pedal tothe floor, the output torque of the engine 21 only rises to the upperlimit torque which has been imposed, for example 80% of full torque, sothat to this extent fuel is economized. By the way, in this torquevariation curve, reduction of the output torque which is temporarilyengendered when the transmission 23 is changed over between forwardtraveling and reverse traveling is caused by the driver temporarilyreleasing the stepping on of the accelerator pedal. Furthermore,reduction of the output torque in the latter half of the soil dumpingprocess is caused by the driver releasing the stepping on of theaccelerator pedal after all of the subject material for work has beendumped.

Although, in the above, embodiments of the present invention have beenexplained, these embodiments are only provided by way of example forexplaining the present invention; and it is not intended to restrict therange of the present invention only to these embodiments. The presentinvention may also be embodiment in various other manners, provided thatits gist is not departed from.

For example, the present invention may also be applied to a workingvehicle of a different type, other than a wheel loader. And variationsare possible in the method of detecting the working state, according tothe type of working vehicle to which the present invention is applied.For example, in the case of a hydraulic shovel, it would be acceptableto determine the magnitude of the load which was being applied to thehydraulic shovel by detecting the hydraulic oil pressure of a boomcylinder, an arm cylinder, or a bucket cylinder.

Although in the above described embodiments it was arranged that, in thehigh output mode, it was possible to manifest the rated or the maximumpower output capacity, without imposing any limitation upon the poweroutput of the engine, the method of control is not limited only to thiscase. For example, it would also be acceptable to arrange to obtain, inthe high output mode, a low power output capacity which is slightlysmaller than the maximum power output capacity. Moreover, it would alsobe acceptable to prepare a plurality of engine output modes whose torquecurves have different shapes, and to arrange to make an appropriateselection from those modes according to the current working state.

Furthermore, instead of, or in addition to, the above described controlin which selection is performed between a plurality of engine outputmodes which are prepared in advance, it would also be acceptable tocontrol the variation of the power output capacity of the enginecontinuously, in other words in a stepless manner, by the controller 50calculating the power output capacity so as to match the size of theload, or the working condition, which has been detected.

1. A wheel loader that is provided with a work apparatus and a travelapparatus that are driven by a power output of an engine and thatcarries out the excavation and loading work by operating the workapparatus while traveling by the travel apparatus, comprising: one ormore detectors which detect one or more of variable values whichindicate a state or states of one or more work loads which consume apower output from an engine; and a controller which controls an upperlimit output torque of said engine that said engine can output accordingto an engine rotational speed, based upon said detected one or morevariable values which have been detected by said one or more detectors,wherein said one or more work loads include a work apparatus which isprovided to said wheel loader, wherein said one or more detectorsinclude a hydraulic oil pressure detector which detects a hydraulic oilpressure or pressures of one or more hydraulic cylinders for drivingsaid work apparatus, and wherein said controller determines whether ornot excavation is being performed based upon said one or more variablevalues, and controls said upper limit output torque of said engine sothat said upper limit output torque for when it is determined that saidexcavation is not being performed is lower than said upper limit outputtorque for when it is determined that said excavation is beingperformed.
 2. The wheel loader according to claim 1, wherein said one ormore detectors include a work apparatus position detector which detectsa position or an attitude of said work apparatus with respect to areference value.
 3. The wheel loader according to claim 1, wherein saidone or more work loads include a travel apparatus which is provided tosaid wheel loader.
 4. The wheel loader according to claim 1, whereinsaid one or more hydraulic cylinders which drive the work apparatus areone or more lift cylinders of the wheel loader, and wherein said one ormore detectors detect the hydraulic oil pressure or pressures at bottomends of said one or more lift cylinders of the wheel loader.